The recording under the bridge

To record the echo, we needed a short sound followed by silence. Traffic above the bridge kept interrupting the quiet interval. Water splashed against one microphone, so we moved it higher and changed the angle of the recording. That made the second attempt difficult to compare with the first. A train then passed nearby, masking the faintest part of the sound. We had agreed to leave before dusk, and the useful gaps between vehicles were growing shorter as the evening traffic increased.

Describe the difficulties of making comparable echo recordings. You may support your points with brief quotations.